Conventionally, a sliding member such as a piston ring in an internal-combustion engine is required to have high sliding performance on a sliding surface thereof, and a Cr plating or a chromium nitride coating film has been used therefor. However, in recent years, the burden on the surface of the sliding member has been increasing due to an increase in output, countermeasures against exhaust gas, and further to the light-weighting of the internal-combustion engine. To address the increase in burden on the sliding member, however, the hard CR plating has insufficient wear resistance, and the chromium nitride coating film has low toughness in spite of its high hardness and excellent wear resistance. Thus, in the case where the sliding environment in the engine is aggravated, there is a problem in that the chromium nitride coating film may be cracked or peeled during use. Accordingly, there is a demand for the development of a chromium nitride coating film which is less likely to be cracked and peeled even under severer conditions.
Patent Literature 1 proposes a coating film having a composition in which one element selected from the group consisting of carbon, phosphorus, nitrogen, boron, and silicon is dissolved in metallic chromium so that the coating film has high hardness, hydrogen brittleness, toughness, and fatigue resistance. Patent Literature 2 proposes a piston ring exhibiting wear resistance and a seizure-resistant characteristic, in which a sliding surface thereof is coated with a coating film having a composition of a mixture containing metallic chromium and chromium nitride (Cr2N).